In general, drugs that are slightly water-soluble at pH 6 to 8 are slightly absorbed into the digestive tract after oral administration because of their low dissolution rates. In order to improve the solubility of the drugs, the slightly water-soluble drugs have been formulated as readily water-soluble salts, or a solution adjuvant is introduced as an additive. However, in the case of hydrochloric acid salts for example, hydrochloric acid sometimes separates from the hydrochloric acid salt during storage. When a solution adjuvant is added, the resulting tablets contain so many ingredients and sometimes become so large that it becomes difficult to take them. In addition, because the acid that is added as an additive readily dissolves and disappears, the effects of improving the solubility also disappear.
Pharmaceutical Research, Vol. 8, No. 1, p. 101 (1991) reports studies on nanocapsules which contain the slightly water-soluble drug indomethacin. However, indomethacin is not readily soluble in water at a pH no higher than 3. In addition, no improvement in the absorption of the drug has been achieved in the study.